A. Waldorf

Determination of optical constants of thin film coating materials based on inverse synthesis

A versatile method for determination of the optical constants is described that can be applied to a variety of coating materials. It is based on the use of an optical thin film synthesis program to adjust the constants of dispersion equations until a good fit is obtained between measured and calculated spectral transmittance and/or reflectance curves. The sensitivity of the determination can be increased by a suitable combination of measurement quantities. Because more than the minimum amount of data can be used, sensitivity to measurement errors and the chances of obtaining multiple solutions can both be reduced. To illustrate the method optical constants are determined of MgF2, ZnS, MgO, Inconel, and Si films in the visible part of the spectrum and of ITO films in the 0.4–12.0-μm range.

Optical coatings deposited by reactive ion plating

The effect of different reactive ion-plating process parameters on the transmittance and the reflectance of single layers of HfO(2), Ta(2)O(5), and SiO(2) are investigated. The optical constants obtained for these three as-deposited materials are presented. Laser-damage threshold trends are examined on single- and double-layer coatings at 1064 nm and on high-reflectance coatings for 248 nm. Single- and double-cavity filters are constructed for the UV (< 1-nm bandwidth) and near-infrared (50-nm bandwidth) regions, respectively. After the filters are postannealed in air at 375 °C for several hours, a shift in the peak wavelengths is observed along with a substantial increase in the peak transmittance. As expected, no significant wavelength shifts result from changes in the humidity of the ambient atmosphere.

Research on thin film anticounterfeiting coatings at the National Research Council of Canada

After a brief description of the threats which face the issuers of documents and valuable papers, the concept of thin film anticounterfeiting coatings proposed by the NRCC is explained. The colorimetric considerations which govern the design of such devices are reviewed. Typical coating designs suitable for use in transmission and reflection are discussed. Reflection coatings can be constructed with and without an integral black absorber coating. The need for geometric patterns in the anticounterfeiting coating is explained. Various methods for producing such logos are proposed. Mention is made of the various numerical tools developed for the design and investigation of single films and multilayers, their desensitization to construction errors, and for the investigation of different process control strategies. During the duration of the project, experiments were performed on the optical constants and nonoptical properties of a number of metal and dielectric films considered for use in the anticounterfeiting coatings. These and some of the processes and equipment investigated during the development of a prototype production facility are reviewed. Finally, some security and economic aspects of the devices are considered.

Transparent, conducting indium tin oxide films formed on low or medium temperature substrates by ion-assisted deposition

An ion-assisted evaporation process is described for the formation of ITO films on substrates that cannot be heated beyond 150 degrees C. Refractive indices and absorption coefficients were measured in the visible part of the spectrum for some of the films. The resistivity of the films varied between 0.7 and 8 x 10(-3) Omega cm, depending on the process parameters. The properties were uniform over 25-cm diam areas.

Graded-reflectance mirrors for beam quality control in laser resonators

Several types of small-dimension graded-reflectance mirrors deposited through rotating masks are compared. Multilayer mirrors provided with single-variable-thickness layers have limitations that are avoided when all the layers in the system are shaped. High-reflectance mirrors of the latter type are demonstrated. Numerical and experimental results are given.

Investigation of the evaporation process conditions on the optical constants of zirconia films

Deposition parameters required for producing zirconia films for use in optical multilayer systems by electronbeam gun evaporation of zirconia and zirconium starting materials were investigated. The optical constants were determined as a function of distance, partial pressure of oxygen, and angle of incidence. The direct and reactive evaporation processes yielded ZrO(2) films with refractive indices of 2.08 and 2.14, respectively, for vapor incident on the substrate at normal incidence.

Practical magnetron sputtering system for the deposition of optical multilayer coatings

A magnetron sputtering system is described in which, at any one time, as many as four different 15-cm x 46-cm rectangular planar magnetron targets can be mounted vertically in the deposition chamber. These can be attached to either dc or rf power supplies for direct or reactive deposition of metal, metal oxide, or nitride films. Typical target materials include Ag, Al, C, Mo, Nb, Ni, Si, W, and Zr. Good uniformity can be obtained on stationary substrates, although better results are possible with oscillating substrates. The refractive indices are given for several useful oxide materials. The materials and thicknesses of the individual layers that comprise an optical multilayer system are entered into a computer that subsequently controls the deposition parameters, the substrate motion, and the deposition time. After a relatively simple calibration process, coatings that consist of between 20 and 60 layers can be produced to within an accuracy of 1% or 2%. A wideband optical monitor is available for checking the performance of the multilayer system during its deposition. Several examples of multilayer coatings that were prepared on this equipment are given.

High-performance thin film polarizer for the UV and visible spectral regions.

The practical limitations on the performance of a conventional MacNeille prism polarizer can be overcome if a liquid is used as the prism material. In particular it is possible to construct large aperture polarizers suitable for use with high power lasers in the UV, visible and near IR spectral regions. The width of the effective spectral region and the polarizing efficiency of the polarizer can be controlled almost at will. The losses in the transmitted beam can be low. The values of the optical thicknesses of the layers are not very critical and the cost of the device is low compared to that of any other polarizer having the same aperture and performance. Results will be given of numerical calculations and of experimental measurements.

Beam splitter for a wide-angle Michelson Doppler imaging interferometer

The design and construction of a hexagonal interferometer beam splitting prism is described in which the light falls onto the all-dielectric partially reflecting coating at an angle of 300 with respect to the normal. For a beam that undergoes one transmission and one reflection by the device, the average effective transmittance 1/2 (T(parallel)R(parallel) + T(perpendicular)R(perpendicular)) in the 0.55-0.75-microm spectral region is approximately 0.23. This quantity is not strongly dependent on the plane of polarization of the incident light, nor on small changes of the angle of incidence from the design angle of 30 degrees. Ways to improve further the performance of the beam splitter are indicated. Coating designs for other angles of incidence are also given.

High performance optical wavelength multiplexer–demultiplexer

The principle of an optical wavelength multiplexer-demultiplexer is described in which the signals undergo repeated reflections from special filter elements that can be designed for a wide range of cross-talk ratios. The insertion losses of these units can be quite small and they can be implemented to provide simultaneous multichannel two-way transmission. In a preliminary investigation of an experimental prototype an insertion loss of 0.5 dB and a cross talk of -35 dB were demonstrated. The multiplexer-demultiplexer is expected to have a long life and high reliability.